The present invention relates to a hydraulic damper or shock absorber for use in a suspension system such as the suspension systems used for automotive cars and trucks. More particularly, the present invention relates to a hydraulic damper or shock absorber which has a two-stage damping characteristic where a relatively low level damping is provided over a specified portion of the stroke of the hydraulic damper or shock absorber and a relatively high level of damping is provided outside of the specified portion of the stroke of the hydraulic damper or shock absorber.
A conventional prior art mono-tube hydraulic damper or shock absorber comprises a cylinder defining a working chamber having a piston slidably engaging the cylinder within the working chamber. The piston thus divides the working chamber into an upper working chamber and a lower working chamber. A piston rod is connected to the piston and it extends out through one end of the cylinder. A first valving system is incorporated into the piston for generating a damping force during the extension stroke of the piston and a second valving system is incorporated into the piston for generating a damping force during the compression stroke of the piston. In a dual tube hydraulic damper or shock absorber, a reservoir tube surrounds the pressure tube to define a reserve chamber. A base valve assembly controls fluid flow between the working chamber and the reserve chamber. A first valving system is incorporated into the piston for generating a damping force during the extension stroke and a second valving system is incorporated into the base valve assembly for generating a damping force during the compression stroke of the piston. The piston includes a valve system to regulate the pressure drop across the piston for fluid flow during the compression stroke and the base valve assembly includes a check valve for fluid flow during the extension stroke.
Various types of damping force generating devices have been developed to generate a variety of desired damping forces in relation to various operating characteristics such as the speed and/or the displacement of the piston within the cylinder. These multi-force damping force generating devices have been developed to provide a relatively small or low damping force during the normal running of the vehicle and a relatively large or high damping force during maneuvers requiring extensions or large suspension movements. The normal running of the vehicle is accompanied by relatively small or fine vibrations of the unsprung mass of the vehicle and thus, the need for a soft ride or low damping characteristic of the suspension system to isolate the sprung mass from these vibrations. During a turning or braking maneuver, as an example, the sprung mass of the vehicle will attempt to undergo a relatively slow and/or large vibration which then requires a firm ride or high damping characteristics of the suspension system to support the sprung mass and provide a stable handling characteristic for the vehicle. Thus, these multi-force damping force generating devices offer the advantage of a smooth steady ride by eliminating the transmission of the high frequency/small amplitude vibrations between the unsprung mass and the sprung mass while still providing the necessary high damping or firm ride for the suspension during vehicle maneuvers causing larger excitations of the sprung mass to provide stability to the sprung mass.
The continued development of hydraulic dampers includes the development of multi-force damping force generating devices which are simpler to manufacture, can be manufactured at a lower cost and which improve the desired force generating characteristics.
The present invention provides the art with a multi-stage hydraulic damper or shock absorber that provides different levels of damping based upon the position of the piston with respect to the pressure tube of the damper. The multi-stage damping is provided by the incorporation of twin pistons and the incorporation of a plurality of by-pass notches formed into the pressure tube wall. The plurality of notches allow fluid flow around one of the two pistons but not around both of them. Thus, when the by-passed piston is in engagement with one or more of the bypass notches, a relatively low damping force is generated. When the by-passed piston is not in engagement with any of the by-pass notches, a relatively high damping force is generated. By appropriately positioning the plurality of by-pass notches within the pressure tube, a relatively low damping force can be generated at typical vehicle heights while still allowing the hydraulic damper or shock absorber to generate a relatively high damping force when the shock absorber travels outside the typical vehicle height.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.